The present invention relates to a method for forming a film on a planar surface, a device for forming a film on a planar surface, a method in rebuilding a converting line comprising means for forming a film on a surface of a sheet-like substrate and a multilayer sheet-like product comprising a film layer. The term planar surface means in this application either a surface of a sheet-like substrate or a counter surface on which surface the film is formed and peeled off after forming. In other words, the latter product is a film without the substrate. The sheet-like substrate means that the substrate is of a sheet material, for example paper, polymeric film or metal film. The substrate can be either in a sheeted form or as a continuous web, preferably it is as a continuous web. The counter surface can be for example an endless belt on which surface the film is formed.
The known methods for forming a film include processes for forming a continuous film web, such as the extrusion process, or processes for forming a film on a sheet-like substrate, such as suspension coating, solvent-base coating and extrusion coating (and lamination). The suspension coating can be used for production of barrier coatings and intermediate layers for wet and dry lamination. In spread coating the coat weight (or applied amount) is adjusted on the web by doctoring, and by roll application the coat weight can be adjusted prior to the application (e.g. gravure applicator). The web requires drying afterwards.
Wet lamination, in which water-based suspensions with dry contents of 40-50% are mostly used, can be utilized to produce laminates such as aluminum foil/adhesive/paper or plastic film/adhesive/paper. Typical adhesives are casein, sodium silicate, starch, and latex (PVAc+EVAc, SB, PE or acrylates).
In dry lamination the water-based adhesive suspension is pre-dried on the web and then laminated with heat. Typically used polymers are hardening (curable) polyurethanes, PVDC, modified SB and acrylic copolymers.
Important properties of polymer dispersion films are barrier properties, for instance the film shall not permeate water or steam, odor, taste, grease, fat, oil, gases such as oxygen, light, or radiation, such as UV radiation. Other required properties may be that it shall be printable, durable against rubbing, puncture and chemicals, and heat sealable. The polymer dispersion films are used for different packages (food/non-food) and wrappings (paper wrappings, bags and kraft paper sacks).
Solvent-base coating can also be used for preparation of wet and dry laminated webs. The process requires solvent evaporation, recovery and recirculation.
The solvent acts as a plasticizer within the polymer during drying, which will change the polymer crystal structure, often weakening its barrier properties. Additionally, there is a risk for residual solvent in the product. Solvent-base coating is used especially for PVDC-based copolymers and restrictedly for coating plastic films, e.g. PET.
The extrusion process involves melting and application of a thin polymer film onto the substrate or between webs. Multilayer application and double-sided coating are possible. The layer thickness produced at a time is approx. 5-100 μm.
The most common coatings are PE-LD, acid-modified copolymers, or polyethylene blends with thermosets or ionomers, lonomers are especially used on metal surfaces.
Important properties of extrusion coated products are adhesion to the substrates, odor and taste, pinhole-free structure, no curling, barrier properties, heat sealability and coefficient of friction. The extrusion coated films are used in liquid packaging, other rigid packaging (folding cartons, cup boards, ovanable boards), industrial applications (wrapping for paper reels and sawn timber, reel end discs, ream wrappers, paper sacks, building materials), flexible packaging (food, also pet food), photographic papers (phototypesetting, graphic arts and monochrome photography).
There are several defects, which relate to the techniques of the prior art. The defects of the prior art are discussed together with the advantages of the present invention.